1. Field of the Invention
The present invention relates to a precision balance for weighing flat filters in a weighing dish, wherein the flat filters rest partially on the weighing dish during weighing.
2. Description of the Related Art
For the determination, for example, of the quantity of solid contaminants contained in gases, particularly in exhaust gases of internal combustion engines or in the exhaust air of industrial plants, flat filters are arranged in the exhaust gas or exhaust line and the solids retained by the filter are weighed. The filters are usually mounted along the rim thereof in a frame which serves as a support device. Consequently, no residues are deposited on the rim portion of the filter which is covered by the frame.
Germany utility model G 8524163.6 discloses a weighing dish for a top-loading balance which is used for weighing flat filters. This known weighing dish has an approximately rectangular plane center portion, wherein upwardly arched corrugations which extend parallel to the edges of the center portion are provided for supporting the flat filter placed on the weighing dish. In addition, in the known weighing dish, the two longer edges thereof form upwardly angled rim portions.
A flat filter placed on the known weighing dish touches with its rim the angled rim portions of the weighing dish. The central portion of the filter on which the residues are deposited comes into contact with the corrugations arranged on the bottom of the weighing dish.
Thus, in the known weighing dish, it is impossible to prevent filter residues from remaining on the corrugations after weighing. This is a disadvantage, particularly when flat filters with different residues must be weighed, or when the residues include radioactive substances. It is difficult to clean the weighing dish because, due to the extremely high resolution of such microbalances, the weighing dish must be constructed with very little mass.